Method for radial tracking in an optical disc drive

ABSTRACT

A method for radial tracking in an optical disc drive ( 1 ) is described. A DTD tracking error signal (S 3 ) is derived from the wobble-induced signal components (W A , W B , W C , W D ) of the optical detector signal (S R ). This tracking error signal is relatively insensitive to beamlanding errors, and to differences in the signal amplitudes K of the output signal of individual detector segments. Further, the need for a 3-spot grating is eliminated. 
     A distinction is made between on the one hand a situation where the track being followed is empty and on the other hand a situation where the track being followed is written. In case the track being followed is empty, a DTD tracking error signal is derived from the wobble-induced signal components of the optical detector signal, whereas, in case the track being followed is written, a DTD tracking error signal is derived from the data-induced signal components of the optical detector signal.

This is a divisional of U.S. Ser. No. 10/557,636, filed Nov. 17, 2005and is incorporated by reference herein.

FIELD OF THE INVENTION

The present invention relates in general to a disc drive apparatus forwriting/reading information into/from an optical storage disc;hereinafter, such disc drive apparatus will also be indicated as“optical disc drive”.

BACKGROUND OF THE INVENTION

As is commonly known to persons skilled in the art, an optical storagedisc comprises at least one track, either in the form of a continuousspiral or in the form of multiple concentric circles, of storage spacewhere information may be stored in the form of a data pattern.

For writing information into the storage space of the optical storagedisc, or for reading information from the disc, an optical disc drivecomprises, on the one hand, rotating means for receiving and rotating anoptical disc, and on the other hand optical means for generating anoptical beam, typically a laser beam, and for scanning the storage trackwith said laser beam.

For optically scanning the rotating disc, an optical disc drivecomprises a light beam generator device (typically a laser diode), anobjective lens for focussing the light beam into a focal spot on thedisc, and an optical detector for receiving the reflected lightreflected from the disc and for generating an electrical detector outputsignal.

During operation, the focal spot should remain aligned with a track orshould be capable of being positioned with respect to a new track. Tothis end, at least the objective lens is mounted radially displaceable,and the optical disc drive comprises radial actuator means forcontrolling the radial position of the objective lens.

The electrical detector output signal contains information on thetracking error, i.e. the radial distance from the centre of the focalspot to the centre of the track being followed. This electrical detectoroutput signal is received by a control circuit, which processes theelectrical detector output signal in order to generate a control circuitfor the radial actuator.

One well-known method to process the electrical detector output signalis to generate a push/pull signal. The push/pull method has somedisadvantages.

One disadvantage of the push/pull method is the sensitivity tobeamlanding errors, i.e. a displacement of the light spot with respectto the optical detector.

A well-known solution to this problem is the three-spot push/pullmethod. Although the three-spot push/pull method solves thebeamlanding-sensitivity problem of the one-spot push/pull method, itintroduces other disadvantages. For one, it is necessary to use hardwareequipment for generating three spots, i.e. a three-spot grating, whichalso needs to be aligned; this adds to the complexity and costs of theoptical system. Further, a three-spot grating effectively splits onelaser beam into three, namely one main beam and two auxiliary beams,resulting in a reduced light intensity of the main beam.

U.S. Pat. No. 6,388,964 discloses a tracking method where a trackingerror signal is generated from the detector output signal on the basisof a differential phase detection method. The method as described inthis document applies to ROM-drives, i.e. applies to discs containingdata. This means that the method as disclosed in said document can notbe applied directly in a drive suitable for handling writable discs,because writable discs may have tracks without data.

SUMMARY OF THE INVENTION

It is a general purpose of the present invention to provide a new radialtracking method as alternative to the push/pull tracking methods.Specifically, the present invention aims to provide a radial trackingmethod which can be used as an alternative to the three-spot push/pullmethod, having the advantages of the three-spot push/pull method withouthaving the disadvantages thereof.

Particularly, the present invention aims to provide a radial trackingmethod which is less sensitive, ideally insensitive, to beamlandingerrors.

Further, the present invention aims to provide a radial tracking methodwhich can be implemented with a simplified optical system without theneed for a three-spot grating.

Further, the present invention aims to provide a radial tracking methodwhich can be applied to discs having tracks without data.

According to an important aspect of the present invention, a trackingerror signal is generated on the basis of a wobble signal. Thus, themethod of the present invention can be applied in all cases of writablediscs which have a wobbled pre-groove. The wobble signal is availableeven if the track is empty.

It may be that, in practice, the tracking method is affecteddisadvantageously if the track is not empty, i.e. if the track containsdata. In such case, according to a further important aspect of thepresent invention, a tracking error signal is preferably generated onthe basis of a data signal.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects, features and advantages of the presentinvention will be further explained by the following description of apreferred embodiment of a disc drive apparatus according to the presentinvention with reference to the drawings, in which same referencenumerals indicate same or similar parts, and in which:

FIG. 1 schematically illustrates an optical disc drive apparatus;

FIG. 2 schematically illustrates an optical detector in more detail;

FIG. 3 is a graph illustrating a DTD error signal as a function of theradial scanning variable;

FIG. 4 is a block diagram illustrating relevant components of a controlcircuit;

FIGS. 5A and 5B are graphs illustrating a frequency spectrum of adetector output signal;

FIG. 5C is a graph illustrating the behaviour of a characteristicfeature of the detector output signal as a function of time;

FIG. 6 is a block diagram illustrating an embodiment of a controllablefilter;

FIG. 7 is a block diagram illustrating an alternative embodiment of anadaptive signal processing;

FIG. 8 is a block diagram illustrating an embodiment of a delaycalculator.

DESCRIPTION OF THE INVENTION

FIG. 1 schematically illustrates an optical disc drive apparatus 1,suitable for storing information on or reading information from anoptical storage disc 2, typically a DVD or a CD or a BD (Bluray Disc).The optical disc 2 comprises at least one track, either in the form of acontinuous spiral or in the form of multiple concentric circles, ofstorage space where information may be stored in the form of a datapattern. The optical disc may be read-only type, where information isrecorded during manufacturing, which information can only be read by auser. The optical disc may also be a recordable (R) or rewritable (RW)type, where information may be stored by a user. Recordable discs arewrite-once discs, where information can only be written once, whereasrewritable discs are write-many discs, where the information contentscan be changed by writing over previously written data.

The present invention relates particularly to writable discs, indicatedas R/RW discs, and thus relates particularly to R/RW disc drives, i.e.disc drives capable of reading and/or writing R/RW discs; examples ofsuch discs are: CD-R, CD-RW, DVD-RW, DVD+RW, DVD+R, BD+RW. Therefore,the present invention will hereinafter specifically be explained forR/RW disc drives. However, it is explicitly noted that the reference toR/RW disc drives is by way of example only, and that it is not intendedto restrict the scope of the present invention in any way to suchexample, because the gist of the present invention is also applicable toread-only discs. Particularly, the present invention is applicable todisc drives which are only capable of reading discs, whether it be awritable disc or not. Also, the present invention is applicable to discdrives which are only capable of reading read-only discs.

Since the technology of optical discs in general, the way in whichinformation can be stored in an optical disc, and the way in whichoptical data can be read from an optical disc, is commonly known topersons skilled in this art, it is not necessary here to describe thistechnology in more detail.

For rotating the disc 2, the disc drive apparatus 1 comprises a motor 4fixed to a frame (not shown for sake of simplicity), defining a rotationaxis 5. For receiving and holding the disc 2, the disc drive apparatus 1may comprise a turntable or clamping hub 6, which in the case of aspindle motor 4 is mounted on the spindle axle 7 of the motor 4.

The disc drive apparatus 1 further comprises an optical system 30 forscanning tracks of the disc 2 with an optical beam. The optical system30 comprises a light beam generating means 31, typically a laser such asa laser diode, arranged to generate a light beam 32. In the following,different sections of the optical path of the light beam 32 will beindicated by a character a, b, c, etc added to the reference numeral 32.

The light beam 32 passes a beam splitter 33, a collimator lens 37 and anobjective lens 34 to reach (beam 32 b) the disc 2. The light beam 32 breflects from the disc 2 (reflected light beam 32 c) and passes theobjective lens 34, the collimator lens 37 and the beam splitter 33 (beam32 d) to reach an optical detector 35.

The objective lens 34 is designed to focus the light beam 32 b in afocal spot F on an information layer (not shown for sake of simplicity)of the disc 2.

During operation, the focal spot should remain aligned with a track orshould be capable of being positioned with respect to a new track. Tothis end, at least the objective lens 34 is arranged radiallydisplaceable, and the optical disc drive apparatus 1 comprises a radialactuator 51 arranged for radially displacing the objective lens 34 withrespect to the disc 2. Since radial actuators are known per se, whilefurther the design and operation of such radial actuator is no subjectof the present invention, it is not necessary here to discuss the designand operation of such radial actuator in great detail.

The disc drive apparatus 1 further comprises a control circuit 90 havingan output 93 coupled to a control input of the radial actuator 51, and aread signal input 91 for receiving a read signal S_(R) from the opticaldetector 35. The control circuit 90 is designed to generate at itsoutput 93 a control signal S_(CR) for controlling the radial actuator51.

FIG. 2 illustrates that the optical detector 35 comprises a plurality ofdetector segments, in this case four detector segments 35 a, 35 b, 35 c,35 d, capable of providing individual detector signals A, B, C, D,respectively, indicating the amount of light incident on each of thefour detector quadrants, respectively. A centre line 36, separating thefirst and fourth segments 35 a and 35 d from the second and thirdsegments 35 b and 35 c, has a direction corresponding to the trackdirection. Since such four-quadrant detector is commonly known per se,it is not necessary here to give a more detailed description of itsdesign and functioning.

FIG. 2 also illustrates that the read signal input 91 of the controlcircuit 90 actually comprises four inputs 91 a, 91 b, 91 c, 91 d forreceiving said individual detector signals A, B, C, D, respectively. Thecontrol circuit 90 is designed to process said individual detectorsignals A, B, C, D, in order to derive data and control informationtherefrom, as will be clear to a person skilled in the art. Forinstance, a data signal S_(D) can be obtained by summation of allindividual detector signals A, B, C, D according to

S _(D) =A+B+C+D  (1)

Further, a push-pull tracking error signal S_(TE) can be obtained bysummation of the signals A and D from all individual detector segments35 a and 35 d on one side of the centre line 36, summation of thesignals B and C from all individual detector segments 35 b and 35 c onthe other side of the centre line 36, and taking the difference of thesetwo summations, according to

S _(TE)=(A+D)−(B+C)  (2a)

In order to compensate light intensity variations of the beam as awhole, this error signal can be normalised by division by the datasignal to obtain a normalised tracking error signal RES, according to

RES=S _(TE) /S _(D)  (2b)

In a case of a disc having tracks in the form of wobbled pregrooves, asis known per se, the four signals A, B, C, D will contain a signalcomponent having a frequency equal to the linear scanning speed dividedby the wobble period. This frequency is the same for all four signals A,B, C, D, but in general the four oscillations are not in phase.

The wobble-induced signal components will be indicated as W_(A), W_(B),W_(C), W_(D), respectively, and can mathematically be written as

W _(A) =K _(A)·cos(τ−τ_(A))  (3a)

W _(B) =K _(B)·cos(τ−τ_(B))  (3b)

W _(C) =K _(C)·cos(τ−τ_(C))  (3c)

W _(D)=K_(D)·cos(τ−τ_(D))  (3d)

wherein K_(A), K_(B), K_(C), K_(D) are the respective amplitudes and

wherein τ_(A), τ_(B), τ_(C), τ_(D) are the respective phases.

τ=2πx/1_(w) is the tangential scanning variable, wherein x representsthe track distance travelled by the beam 32, and wherein 1_(w)represents the wobble period.

It can be shown that the respective phases depend on the radial scanningvariable φ=2πy/t_(p), wherein t_(p) represents track pitch, and whereiny represent the radial error, i.e. the radial distance between thecentre of the spot and the centre of the track.

For any pair of signals P, Q (P and Q representing A, B, C, D), themutual delay Δ(P,Q) can be defined as

Δ(P,Q)=τ_(P)−τ_(Q)  (4)

A preferred suitable tracking error signal derived from thewobble-induced signal components is defined as

DTD4R=Δ(A,B)+Δ(C,D)  (5)

FIG. 3 is a graph showing this preferred tracking error signal DTD4Robtained by a numerical simulation with DVD+RW pregroove parameters. Thehorizontal axis represents the radial scanning variable φ, while thevertical axis represent DTD4R. It can be seen from FIG. 3 that DTD4R isproportional to φ in a range of φ around zero, so that DTD4R can indeedbe used as tracking error signal.

It is noted that in the simulation it was assumed that two neighbouringtracks have their respective wobbles mutually in phase. Normally, thiswill not be the case in practice, which may lead to a different shape ofthe error signal DTD4R. Nevertheless, in a range of φ around zero, DTD4Rremains proportional to φ to a good approximation, so that DTD4R canstill be used as tracking error signal.

The DTD4R signal as defined by formula 5 is not the only signalpotentially capable of functioning as tracking error signal. In analternative embodiment, a signal DTD4T is used, defined as

DTD4T=Δ(A,D)+Δ(C,B)  (6)

However, when comparing the DTD4R and DTD4T signals, the DTD4R signal ispreferred because it introduces less noise in the final error signalthan the DTD4T signal.

In another alternative embodiment, a signal DTD2 is used, defined as

DTD2=Δ(A+C,B+D)  (7)

However, when comparing the DTD4R and DTD2 signals, the DTD4R signal ispreferred because it introduces less noise in the final error signalthan the DTD2 signal. Further, the DTD2 signal is more sensitive tobeamlanding errors.

The method as described above, i.e. the use of a wobble-derived signalas tracking error signal, works well if a track is empty, i.e. for atrack which does not contain any data written in it. If, however, atrack does contain data, the data introduce noise into suchwobble-derived signal. In order to eliminate or at least reduce thisnoise, it is possible to use, at the control circuit input, a suitablefilter which is designed to pass a frequency range comprising a signalcomponent useable as tracking error signal, and to stop a frequencyrange comprising the disadvantageous noise components; indeed such anembodiment is an embodiment within the scope of the present invention.

However, if a track does contain data, it is preferred as tracking errorsignal to use a signal derived from the data signal. Then, thewobble-derived signal becomes an undesired signal component, which canbe eliminated or at least reduced by using a suitable filter which isdesigned to pass a frequency range comprising a data signal componentuseable as tracking error signal, and to stop a frequency rangecomprising the wobble-derived signal.

Thus, in order to be able to handle a disc which contains written tracksas well as virgin tracks, it is desirable to use a controllable filterdevice which is capable of being controlled to have a first filtercharacteristic suitable for use in the case of a written track andcapable of being controlled to have a second filter characteristicsuitable for use in the case of a virgin track.

FIG. 4 is a block diagram schematically showing relevant components of adisc drive according to a preferred embodiment of the invention. In thisembodiment, the control circuit 90 comprises four controllable filterdevices 110 a, 110 b, 110 c, 110 d, having respective outputs 112 a, 112b, 112 c, 112 d, and having respective inputs 111 a, 111 b, 111 c, 111 dcoupled to the respective signal inputs 91 a, 91 b, 91 c, 91 d of thecontrol circuit 90. Further, the control circuit 90 comprises two delaycalculators 120 and 130, having respective outputs 123 and 133.

The first delay calculator 120 has a first input 121 coupled to theoutput 112 a of the first controllable filter device 110 a, and has asecond input 122 coupled to the output 112 d of the fourth controllablefilter device 110 d. At its output 123, the first delay calculator 120provides a signal S1 representing the delay Δ(A,D) between the signals Aand D of the first and fourth detector segments 35 a and 35 d,respectively.

The second delay calculator 130 has a first input 131 coupled to theoutput 112 b of the second controllable filter device 110 b, and has asecond input 122 coupled to the output 112 c of the third controllablefilter device 110 c. At its output 133, the second delay calculator 130provides a signal S2 representing the delay Δ(B,C) between the signals Band C of the second and third detector segments 35 b and 35 c,respectively.

Further, the control circuit 90 comprises a first adder 140, having anoutput 143. The first adder 140 has a first input 141 coupled to theoutput 123 of the first delay calculator 120, and has a second input 142coupled to the output 133 of the second delay calculator 130. At itsoutput 143, the first adder 140 provides a signal S3 representing theDTD4R signal.

Further, the control circuit 90 comprises a second adder 150, havingfour inputs 151 a, 151 b, 151 c, 151 d coupled to the respective signalinputs 91 a, 91 b, 91 c, 91 d of the control circuit 90. At an output152, the second adder 150 provides a signal S4 representing the centralaperture signal CA of the optical detector 35, i.e. the sum signal ofthe four detector quadrants. Further, the control circuit 90 comprises afilter controller 160, having an input 161 coupled to receive the outputsignal S4 of the second adder 150. The filter controller 160 has anoutput 162, coupled to respective control inputs 113 a, 113 b, 113 c,113 d of the controllable filters 110 a, 110 b, 110 c, 110 d.

Alternatively, the filter controller 160 may have four separate outputs162 a, 162 b, 162 c, 162 d (not shown), each coupled to the respectivecontrol inputs of the controllable filters 110 a, 110 b, 110 c, 110 d.

The filter controller 160 is designed to evaluate its input signal todetermine whether or not the current track contains data. In a suitableembodiment, this is done on the basis of the frequency spectrum of thissignal. FIG. 5A is a graph schematically illustrating the shape of thefrequency spectrum 170A of the central aperture signal CA for the caseof an unwritten track, while FIG. 5B is a similar graph showing thefrequency spectrum 170B of the central aperture signal CA for the caseof a written track. In both cases, the horizontal axis representsfrequency in arbitrary units and the vertical axis represents signalpower in arbitrary units.

It is noted that these graphs are only showing idealized contours of thefrequency spectrum, for illustrating some qualitative aspects ingeneral. In reality, such spectra have a more complicated shape, as willbe understood by persons skilled in this art.

In the following, the index A and B, respectively, to a referencenumeral will be used to specify the case of an unwritten track and thecase of a written track, respectively, whereas the reference numeralwithout such index will be used to indicate the corresponding feature inany case.

When comparing FIGS. 5A and 5B, it can be seen that the frequencyspectrum 170 always contains a first significant peak 171 in a lowfrequency range of approximately 0 to 100 or 1000 Hz. This peak 171 willbe indicated by the phrase “DC-peak”. Further, it can be seen that theheight of the DC-peak 171B for the case of a written track issignificantly lower than the height of the DC-peak 171A for the case ofan unwritten track.

Further, the frequency spectrum 170 always contains a second significantpeak 172 in a range around the wobble frequency, which typically is inthe range of 1 MHz in case of a 1×DVD+RW system. This peak 172 will beindicated by the phrase “wobble-peak”. When comparing FIGS. 5A and 5B,it can be seen that the height of the wobble-peak 172 is substantiallyunaffected by the presence or absence of data.

Further, in the case of a written track, the frequency spectrum 170Bcontains a third significant peak 173B in the range corresponding todata frequencies, typically in the range of 1-10 MHz in case of a1×DVD+RW system. This third significant peak will be indicated by thephrase “data-peak”. Since an unwritten track does not contain data, thefirst frequency spectrum 170A does not contain such data-peak.

The filter controller 160 may be designed to use any of theabove-mentioned, or possible other, differences to decide whether or notthe current track contains data, and to generate at its output 162 afilter control signal S_(FC) of which the value depends on the outcomeof this decision such as to switch the filter characteristics of thecontrollable filters 110. For instance the filter control signal S_(FC)may have a first value (e.g. a high level or a digital “1”) if thefilter controller 160 finds that data is present, and it may have asecond value (e.g. a low level or a digital “0”) if the filtercontroller 160 finds that data is not present.

In one embodiment, the filter controller 160 may be designed to monitorthe DC-peak 171 of the CA-signal (or more precisely: to measure thesignal power in a low frequency range), and to compare the height of theDC-peak 171 with a predetermined reference level, indicated at 174 inFIGS. 5A and 5B. If the measured height is above this predeterminedreference level, the filter controller 160 decides that data is absent,whereas if the measured height is below this predetermined referencelevel, the filter controller 160 decides that data is present.

In a second embodiment, the filter controller 160 may be designed tomonitor the data-peak 173 (or more precisely: to measure the signalpower in the frequency range corresponding to data frequencies), and tocompare the height of the data-peak 173 with a predetermined referencelevel, indicated at 175 in FIGS. 5A and 5B. If the measured height isabove this predetermined reference level, the filter controller 160decides that data is present, whereas if the measured height is belowthis predetermined reference level, the filter controller 160 decidesthat data is absent.

The above two embodiments have the characteristic that at any time thefilter controller 160 finds the current status of the track: YES DATA orNO DATA. However, a difficulty may be to define a suitable value for thereference level, especially in the first embodiment. In an alternativeembodiment, based on the same principles as the first embodiment, thefilter controller 160 again is designed to monitor the DC-peak 171 ofthe CA-signal, but, instead of comparing the current height of theDC-peak 171 with a predetermined reference level, the filter controller160 monitors variations of the DC-peak 171. FIG. 5C is a time diagram,illustrating an example of changes in the height of the DC-peak 171 as afunction of time (top graph). Starting at the left, the height of theDC-peak 171 remains substantially constant at a first level H, untiltime t1, when the height of the DC-peak 171 suddenly drops significantlyto a lower level L. The filter controller 160 may take such drop asindicating a transition from an empty track to a written track.

After the drop at time t1, the height of the DC-peak 171 remainssubstantially constant again at said lower level L, until time t2, whenthe height of the DC-peak 171 suddenly rises significantly to the higherlevel H. The filter controller 160 may take such rise as indicating atransition from a written track to an empty track.

It is also possible that the filter controller 160 is designed tocalculate a time-derivative of the DC-peak 171, indicated as lower graph176 in FIG. 5C. Normally, this time-derivative 176 is substantiallyequal to zero. Only at the transition moments t1 and t2, thistime-derivative 176 shows a negative and a positive peak, respectively.For using this signal, the filter controller 160 may be designed tocompare the value of this time-derivative with predetermined negativeand positive threshold levels, indicated at 177 and 178 in FIG. 5C. Aslong as the magnitude of this time-derivative is below such threshold, acurrent status is maintained. If the magnitude of this time-derivativeexceeds any said predetermined threshold levels, the filter controller160 may take such event as indicating a transition from a written trackportion to an unwritten track portion (if the magnitude of thetime-derivative exceeds the negative threshold level 177) or atransition from an unwritten track portion to a written track portion(if the magnitude of the time-derivative exceeds the positive thresholdlevel 178).

Alternatively, instead of the DC-peak 171, the filter controller 160 mayuse the data-peak 173 for finding sudden drops and rises, of forcalculating a time-derivative and to find peaks in such timederivatives. It should be clear that, in this case, a transition from anunwritten track portion to a written track portion is associated with arise of the data-peak 173 and with a time-derivative exceeding apositive threshold level, while a transition from a written trackportion to an unwritten track portion is associated with a drop of thedata-peak 173 and with a time-derivative exceeding a negative thresholdlevel.

FIG. 6 is a block diagram schematically illustrating a possibleembodiment of a controllable filter device 110. In this embodiment, thecontrollable filter device 110 comprises two separate filters 115, 116.The first filter 115 is designed to pass signal components in the rangeof the wobble frequency and to stop signal components in the range ofthe data frequencies, whereas the second filter 116 is designed to stopsignal components in the range of the wobble frequency and to passsignal components in the range of the data frequencies. For instance,the first filter 115 may be a band-pass filter centred at the wobblefrequency, and the second filter 116 may be a high-pass filter, havingits cut-off frequency set at a suitable value between the wobblefrequency and the data frequencies.

Each filter 115, 116 has its input 115 a, 116 a coupled to the input 111of the controllable filter device 110, and filter 115, 116 has itsoutput 115 b, 116 b coupled to a respective input 117 a, 117 b of acontrollable switch 117. The controllable switch 117 has an output 117 ccoupled to the output 112 of the controllable filter device 110. Thecontrollable switch 117 has a control input 117 d coupled to the controlinput 113 of the controllable filter device 110. The controllable switch117 is responsive to the control signal received at its control input117 d to switch between a first operative state where its output 117 cis connected to its first input 117 a and a second operative state whereits output 117 c is connected to its second input 117 b. Thus, dependingon the operative state of the controllable switch 117, either the firstfilter 115 or the second filter 116 is active, which means that thecontrollable filter device 110 as a whole either shows the filtercharacteristics of the first filter or the filter characteristics of thesecond filter 116.

FIG. 7 is a block diagram schematically illustrating an alternativeembodiment of the control circuit, indicated at reference numeral 290.In this embodiment, the control circuit 290 comprises a first branch offilter devices 310 a to 310 d, delay detectors 320, 330, and adder 340,connected in a manner comparable to the circuit described above withreference to control circuit 90 (FIG. 4). These components may beidentical to the components 110, 120, 130, 140 in the above-describedembodiment of control circuit 90, with the exception that the filterdevices 310 a to 310 d do not need to be controlled and therefore do notneed to be controllable filter devices; in fact, these filter devices310 a to 310 d may each be identical to the first filter 115 describedabove.

Further, in this embodiment, the control circuit 290 comprises a secondbranch of filter devices 410 a to 410 d, delay detectors 420, 430, andadder 440, connected in a manner comparable to the first branch.Likewise, these components may be identical to the components 110, 120,130, 140 in the above-described embodiment of control circuit 90, withthe exception that the filter devices 410 a to 410 d are not controlledand therefore do not need to be controllable filter device; in fact,these filter devices 410 a to 410 d may each be identical to the secondfilter 116 described above.

The filter devices 310 a to 310 d and the filter devices 410 a to 410 dhave their respective inputs 311 a-d and 411 a-d connected in parallelto the respective inputs 291 a-d of the control circuit 290. Thus, thefirst adder 340 of the first branch provides the wobble-derived DTD4signal, while the second adder 440 of the second branch provides thedata-derived DTD4 signal.

The control circuit 290 further comprises a controllable switch 299,which has its two inputs coupled to the outputs of the adders 340 and440, respectively, and which is controlled by the controller 160, toswitch between a first operative state where its first input isconnected to its output and a second operative state where its secondinput is connected to its output. So, in this embodiment, too, eitherthe wobble-derived DTD4 signal or the data-derived DTD4 signal is usedas tracking error signal.

While the above description clearly explains the principles of thepresent invention, in practice it may be advantageous to invert one ofthe signals of the signal pairs for which the delay A is calculated,which may result in an improved smoothness of the DTD4 signal in therange around φ=0. Such inversion is equivalent to a delay of π. Thus, ina more general form, formula (5) can be written as:

DTD4R=Δ(A,sB)+Δ(C,sD)  (8)

wherein s is either +1 or −1.

In FIGS. 4 and 7, this functionality is illustrated as a signal S, whichis fed to sign inputs 124, 134, 324, 334, 424, 434 of the delaycalculators 120, 130, 320, 330, 420, 430, respectively. This signal Smay be equal to, or derived from, the output filter control signalS_(FC) from the filter controller 160. In an embodiment, s equals +1 inthe presence of data, and s equals −1 in the absence of data.

FIG. 8 is a block diagram which schematically shows how thisfunctionality can be implemented in a delay calculator, for exampledelay calculator 120. The delay calculator device 120 of this examplecomprises a controllable switch 125, having a first input 125 a coupledto the second device input 122, a second input 125 b, a control input125 c coupled to the device control input 124, and an output 125 d. Thedelay calculator device 120 of this example further comprises aninverter 127, having its input coupled to the second device input 122and having its output coupled to the second input 125 b of thecontrollable switch 125. The controllable switch 125 is designed toconnect its output 125 d to either its first input 125 a or its secondinput 125 b, depending on the value of the signal received at itscontrol input 125 c.

The delay calculator device 120 of this example further comprises anactual delay calculating unit 126, having a first input 126 a coupled tothe first device input 121, a second input 126 b coupled to the output125 d of the controllable switch 125, and an output 126 c coupled to thedevice output 123, which actual delay calculating unit 126 is designedto calculate the delay between the signals arriving at its two inputsand generating an output signal representing this delay.

Thus, the present invention provides a method for radial tracking in anoptical disc drive, wherein a DTD tracking error signal is derived fromthe wobble-induced signal components of the optical detector signal.This tracking error signal is relatively insensitive to beamlandingerrors, and to differences in the signal amplitudes K of the outputsignal of individual detector segments. Further, the need for a 3-spotgrating is eliminated.

In a preferred embodiment, a distinction is made between on the one handa situation where the track being followed is empty and on the otherhand a situation where the track being followed is written. In case thetrack being followed is empty, a DTD tracking error signal is derivedfrom the wobble-induced signal components of the optical detectorsignal, whereas, in case the track being followed is written, a DTDtracking error signal is derived from the data-induced signal componentsof the optical detector signal.

It should be clear to a person skilled in the art that the presentinvention is not limited to the exemplary embodiments discussed above,but that various variations and modifications are possible within theprotective scope of the invention as defined in the appending claims.

For instance, the filter controller may be designed to decide whether ornot the current track contains data on the basis of another criterion.

Further, in the above, the present invention has been explained for acase where an optical detector 35 produces four output signals,corresponding to four detector segments, all four of these signals beingused. However, it is also possible that the optical detector 35 has adifferent number of detector segments, hence produces a different numberof output signals. It is also possible that the tracking error signal isderived from only some of the detector output signals.

In the above, the present invention has been explained with reference toblock diagrams, which illustrate functional blocks of the deviceaccording to the present invention. It is to be understood that one ormore of these functional blocks may be implemented in hardware, wherethe function of such functional block is performed by individualhardware components, but it is also possible that one or more of thesefunctional blocks are implemented in software, so that the function ofsuch functional block is performed by one or more program lines of acomputer program or a programmable device such as a microprocessor,microcontroller, etc.

1. Method for radial tracking in an optical disc drive (1), wherein atracking error signal (S3) is derived from wobble-induced signalcomponents (W_(A), W_(B), W_(C), W_(D)) of an optical detector outputsignal (S_(R)).
 2. Method according to claim 1, wherein the trackingerror signal (S3) is generated according to the formulaDTD4R=Δ(A,B)+(C,D), wherein Δ(A,B) represents the delay τ_(A)−τ_(B)between signals W_(A)=K_(A)•cos(τ−τ_(A)) and W_(B)=K_(B)•cos(τ−τ_(B)),Δ(C,D) represents the delay τ_(C)−τ_(D) between signalsW_(C)=K_(C)•cos(τ−τ_(C)) and W_(D)=K_(D)•cos(τ−τ_(D)), wherein K_(A),K_(B), K_(C), K_(D) are respective amplitudes τ_(A), τ_(B), τ_(C), τ_(D)are respective phases τ is the tangential scanning variable and whereinW_(A), W_(B), W_(C), W_(D) represent the amount of light received atrespective segments of an optical detector (35).
 3. Method according toclaim 1, wherein the tracking error signal is generated according to theformulaDTD4T=Δ(A,D)+(C,B), wherein Δ(A,D) represents the delay τ_(A)−τ_(D)between signals W_(A)=K_(A)•cos(τ−τ_(A)) and W_(D)=K_(D)•cos(τ−τ_(D)),Δ(C,B) represents the delay τ_(C)−τ_(B) between signalsW_(C)=K_(C)•cos(τ−τ_(C)) and W_(B)=K_(B)•cos(τ−τ_(B)), wherein K_(A),K_(B), K_(C), K_(D) are respective amplitudes τ_(A), τ_(B), τ_(C), τ_(D)are respective phases τ is the tangential scanning variable and whereinW_(A), W_(B), W_(C), W_(D) represent the amount of light received atrespective segments of an optical detector (35).
 4. Method according toclaim 1, wherein the tracking error signal is generated according to theformulaDTD2=Δ(A+C,B+D), wherein Δ(A+C,B+D) represents the delay τ_(A+C)−τ_(B+D)between signals W_(A)+W_(C)=K_(A+C)•cos(τ−τ_(A+C)) andW_(B)W_(D)=K_(B+D)•cos(τ−τ_(B+D)), wherein K_(A), K_(B), K_(C), K_(D)are respective amplitudes τ_(A), τ_(B), τ_(C), τ_(D) are respectivephases τ is the tangential scanning variable and wherein W_(A), W_(B),W_(C), W_(D) represent the amount of light received at respectivesegments of an optical detector (35).
 5. Method for radial tracking inan optical disc drive (1), wherein, if the track being followed isnon-written, a tracking error signal (S3) is derived from wobble-inducedsignal components (W_(A), W_(B), W_(C), W_(D)) of an optical detectoroutput signal (S_(R)) in accordance with claim 1, and wherein, if thetrack being followed is written, a tracking error signal (S3) is derivedfrom data-induced signal components of the optical detector outputsignal (S_(R)).
 6. Method according to claim 5, wherein the opticaldetector output signal (S_(R)) is evaluated to determine whether thetrack being followed is non-written or written.
 7. Method according toclaim 6, wherein the determination whether the track being followed isnon-written or written is made on the basis of the signal power contentsof a low-frequency part (171) of the central aperture signal (CA). 8.Method according to claim 7, wherein the signal power contents of thelow-frequency part (171) of the central aperture signal (CA) ismonitored, and wherein, in case a substantial drop in signal power isdetected, tracking error signal generation is switched to deriving thetracking error signal from data-induced signal components whereas, incase a substantial rise in signal power is detected, tracking errorsignal generation is switched to deriving the tracking error signal fromwobble-induced signal components (W_(A), W_(B), W_(C), W_(D)) of anoptical detector output signal.
 9. Method according to claim 6, whereinthe determination whether the track being followed is non-written orwritten is made on the basis of the signal power contents of adata-frequency part (173) of the central aperture signal (CA). 10.Method according to claim 9, wherein the signal power contents of thedata-frequency part (173) of the central aperture signal (CA) ismonitored, and wherein, in case a substantial rise in signal power isdetected, tracking error signal generation is switched to deriving thetracking error signal from data-induced signal components whereas, incase a substantial drop in signal power is detected, tracking errorsignal generation is switched to deriving the tracking error signal fromwobble-induced signal components (W_(A), W_(B), W_(C), W_(D)) of anoptical detector output signal. 11-24. (canceled)